This invention relates to power steering systems for automotive vehicles and, more particularly, to a damping system for a rack and pinion power steering system in an automotive vehicle.
Steering systems on vehicles equipped with a rack and pinion-type steering frequently experience high speed steering wheel shake. Such high speed shake of the steering wheel is detrimental to the feel of the steering to the driver of the vehicle. Prior attempts to reduce or eliminate such high speed shake or vibrations have proven unsuccessful. More particularly, the conventional means of attenuating high speed shake are ineffective on vehicles with rack and pinion-type steering systems. The primary reasons that earlier attempts to reduce or eliminate high speed shake and vibrations of this type have not been effective for rack and pinion-type steering systems are due to the mounting and frequency requirements for such systems.
Previously, linear dampers have been used to dampen steering wheel oscillations or vibrations. Linear dampers are commonly effective on larger vehicles, which have large displacement steering systems, because the linear damper devices focus on the steering system velocity. Rack and pinion-type steering systems typically have a high frequency and yet low amplitude vibration or shake, and linear dampers are not adequately suited to address vibrations of this type. The low amplitude vibration does not provide sufficient velocity for the linear damper to operate effectively.
Other devices which have been used to address high speed steering wheel shake include constant friction-type interfaces. However, such devices have also proven to be unacceptable for rack and pinion-type steering systems because the feel and the return-to-center characteristics of the steering system are detrimentally impacted or destroyed with constant friction-type interfaces.
Therefore, a need exists for a damping system for high speed shake and vibrations of high frequency and low amplitude in rack and pinion-type steering systems while still providing good responsive feeling to the steering wheel and return-to-center characteristics.
This invention addresses the above-described objectives and other objectives associated with vehicle rack and pinion-type steering systems. Specifically, the invention reduces high speed shake by transferring the energy of the motion to a stationary member, such as a rack and pinion housing, a frame, or other solid member associated with the automotive vehicle or steering system. To that end, the present invention links the steering system to the solid, stationary member when the steering wheel is not being turned.
In one embodiment of the invention, the vehicle steering system comprises a steering wheel and a steering shaft which is coupled between the steering wheel and wheels of the vehicle to turn the vehicle wheels when the steering wheel is turned. The vibration damping system which absorbs vibrations in the steering wheel and steering shaft comprises a rotor which is coupled to the steering shaft to rotate with the shaft when the steering wheel is turned. A rigid case surrounds the rotor and a clutch surface is positioned proximate the rotor. When the steering wheel is not being turned, such as to steer the vehicle, the rotor is operable to engage the clutch surface and thereby vibrationally couple the steering column to the case to absorb vibrations in the column. The case may be coupled to some other solid part of the vehicle for dissipating the vibrations. The rotor is further operable, when a magnetic flux is generated therein, to disengage the clutch surface so that the steering column may more freely rotate for turning the vehicle. A magnetic circuit is utilized for selectively generating a magnetic flux in the rotor when the vehicle is to be steered.
More specifically, the rotor of the vibration damping system is formed of a magnetostrictive material. A coil is wrapped around the rotor and selectively generates a magnetic flux therein. The rigid case is formed of a material for containing the magnetic flux proximate the rotor. The clutch surface is on a non-magnetic stator to be engaged by the rotor. The magnetostrictive material, such as nickel, or a turbium/iron alloy changes shape and dimension. That is, upon application of the magnetic flux within the rotor, the rotor is dimensionally modified. Utilizing a cylindrical rotor, as in one embodiment, the cylindrical diameter of the rotor decreases when the magnetic flux is generated therein and the cylinder increases in length. As such, the rotor disengages the clutch surface which surrounds the rotor. The rotor then returns to a generally non-modified dimensional shape to re-engage the clutch surface when the magnetic flux is no longer generated therein.
In operation, when the steering wheel is not being turned, the vibration damping system isolates the high speed shake energy from the driver by transmitting it through the rigid case which is then mounted to another stationary or rigid member. Upon the need to turn the vehicle, an input from the steering wheel, such as an electrical signal generated by a sensor, may be used as an input to release the rotor, such as by triggering operation of the magnetic circuit. Upon generating a flow of magnetic flux through the magnetostrictive rotor material, the rotor contracts to allow rotation of the steering wheel with little or no unwanted side effects, such as excessive drag, poor return-to-center characteristics, or significantly increased steering effort based upon inertial effects of the vibration damping system. These features and other features of the invention become more readily apparent from the detailed description of the invention below.